Mechanically decoupled opto-mechanical connector for flexible optical waveguides embedded and/or attached to a printed circuit board

ABSTRACT

Printed circuit boards that include optical interconnects include a flexible optical waveguide embedded or locally attached to the board having at least one end mechanically decoupled from the board during fabrication that can be fitted with a mechanical connector. Also disclosed are processes for fabricating the circuit board.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of and claims priority to U.S.patent application Ser. No. 11/877,859 filed on Oct. 24, 2007 now U.S.Pat. No. 7,389,015, the entire contents of which are incorporated hereinby reference in its entirety.

TRADEMARKS

IBM® is a registered trademark of International Business MachinesCorporation, Armonk, N.Y., U.S.A. Other names used herein may beregistered trademarks, trademarks or product names of InternationalBusiness Machines Corporation or other companies.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention generally relates to mechanically decoupledopto-mechanical connectors for flexible optical waveguides embeddedand/or attached to a printed circuit board. More particularly, theinvention relates to improvements of an optical interface between anopto-enhanced printed circuit board and associated electronics thereon.

2. Description of Background

Printed circuit boards (PCBs) generally include multiple integratedcircuits mounted upon their surfaces. PCBs typically contain multipleconductive and dielectric layers interposed upon each other, andinterlayer conductive paths (referred to as vias), which may extend froman integrated circuit mounted on a surface of the PCB to one or moreconductive layers embedded within the PCB.

The speed and complexity of integrated circuits are increasing rapidlyas integrated circuit technology advances from very large scaleintegrated (“VLSI”) circuits to ultra large scale integrated (“ULSI”)circuits. As the number of components per chip, the number of chips perboard, the modulation speed and the degree of integration continue toincrease, electrical interconnects are facing fundamental limitations inareas such as speed, packaging, fan-out, and power dissipation.

The employment of optical interconnects will be one of the majoralternatives for upgrading the interconnection speed wheneverconventional electrical interconnection fails to provide the requiredbandwidth. However, the introduction of optics into the PCB causesproblems due to the necessary optical connection, which havesubstantially different requirement s than commonly utilized electricalinterconnects, mechanical connectors, thermal interfaces, and the like.One problem is the proper connection of the waveguides in the board withwaveguides in other boards, with opto-electronic modules on board, andwith test equipment such as fiber bundles. For example, connectionsbetween the opto-electronic subassembly and the PCB would requireelectrical lines with high speed capability to the PCB, an optical pathfor the waveguides that are placed in or on the PCB, mechanicalconnectors between the opto-electronic subassembly and the PCB as wellas a thermal interface to the heat sink. Connections between boardswould require electrical high-speed lines (often, but not only,standardized backplane connectors), optical connections with precisealignment, and rugged mechanical connections. Connection between theboard and the test equipment would require optical connection withprecise alignment and compatibility to standard fiber bundles. Thedifferent connections as noted above and the specific propertiesrequired for the connections, i.e., electrical, thermal (differentcoefficients of thermal expansion between dissimilar materials), and thelike as well as the large tolerances in current PCB manufacturingprocesses, lead to numerous potential problems. For efficient opticalcoupling, the alignment accuracy of multimode polymer waveguides has tobe in the range of 5 to 10 micrometers, whereas current PCB tolerancesare in the range of about 100 micrometers.

Accordingly, there remains a need for improvement of the opticalinterface between an opto-enhanced printed circuit board and associatedelectronics thereon.

SUMMARY OF THE INVENTION

The shortcomings of the prior art are overcome and additional advantagesare provided through the fabrication of a printed circuit boardcomprising a rigid substrate; a flexible optical waveguide locallyattached to the rigid substrate having a least one end mechanicallydecoupled from the rigid substrate and fitted with a mechanicalconnector; and an optical facet in optical communication with themechanical connector of the flexible optical waveguide. The printedcircuit board provides for mechanical decoupling of the opticalconnector from a rigid substrate (i.e., PCB), wherein the electricalconnection between the PCB and the opto-electronic subassembly can berigid or flexible. By way of example, a rigid connection can be anopto-electronic module directly attached with a ceramic carrier that ismounted onto the PCB. A flexible connection is provided that introduceselectrical flex as a connection between the PCB and the opto-electronicmodule. As a result, mechanical stress is minimized and increasedversatility is achieved.

The fabrication process includes providing a rigid support in the regionof the connector where the alignment features, e.g., copper markers, arelocated. Local lamination or local release of the flexible opticalwaveguide is provided by structured adhesive tape, bonding film,acrylate adhesive foil, patterned glue, patterned adhesion inhibitorlayer, a scarification layer or any combination thereof.

Additional features and advantages are realized through the techniquesof the present invention. Other embodiments and aspects of the inventionare described in detail herein and are considered a part of the claimedinvention. For a better understanding of the invention with advantagesand features, refer to the description and to the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter, which is regarded as the invention, is particularlypointed out and distinctly claimed in the claims at the conclusion ofthe specification. The foregoing and other objects, features, andadvantages of the invention are apparent from the following detaileddescription taken in conjunction with the accompanying drawings inwhich:

FIG. 1 illustrates a sectional view of a printed circuit board includinga flexible optical waveguide that is locally attached to the board andhaving a least one end mechanically decoupled from the rigid substrateand fitted with a mechanical connector;

FIG. 2 illustrates a sectional view of an exemplary flexible opticalwaveguide;

FIG. 3 illustrates a process for fabricating the printed circuit boardof FIG. 1.

FIG. 4 illustrates bottom-up views of the printed circuit board aftermilling and laser drilling of the alignment features, and after lasercut to provide release or the flexible optical waveguide; and

FIG. 5 illustrates top down view of the printed circuit board aftertrench milling, optical facet preparation, and flex contour milling.

FIG. 6 illustrates a printed circuit board, the backplane, whereby thereleased, flexible optical waveguide is used to provide a bend in thelight path, e.g. a 90° bend. Therefore, an optical connection to thewaveguides on the second printed circuit broad, the daughter card, isenabled.

DETAILED DESCRIPTION OF THE INVENTION

The detailed description explains the preferred embodiments of theinvention, together with advantages and features, by way of example withreference to the drawings.

Disclosed herein is a circuit board having a mechanically decoupledopto-mechanical connector for coupling a flexible optical waveguide thatis releasably disposed thereon to an opto-electronic subassembly ormodule. As will be discussed in greater detail below, the flexibleoptical waveguide includes an end portion that is releasably attached ata position that is close to the optical facet and is equipped with apassively aligned connector. This results in a mechanically decoupledopto-mechanical connector for the waveguide with minimal mechanicalstress. Any type of optical element, e.g., active or passive, can beconnected to the connector.

FIG. 1 illustrates a partial cross-section of a circuit board 10 thatincludes a flexible optical waveguide shown generally at 12 having oneend 14 adapted to be releasably attached and equipped with a passivelyaligned connector 16. This results in a mechanically decoupledopto-mechanical connector for the waveguide 12. Any kind of opticalelement, active or passive, can be connected to this opto-mechanicalconnector.

The flexible optical waveguide end 14 is disposed in close proximity toan optical facet (not shown) and also includes one or more passivealignment features 17, e.g., a copper marker. The passive alignmentfeature 17 is utilized to align a mechanical adapter 19, which acts asan opto-mechanical termination of the waveguide 12. The circuit board(i.e., PCB) includes a relatively rigid upper board 13, which caninclude various electrical lines including all high frequency lines.Exemplary board materials include, without limitation FR4 (woven glassfiber bundles in a resin matrix), surface laminated circuitry boards,substrate materials used for organic and inorganic carriers, polyimides,LCP, and the like.

A stiffener board 20 is shown attached to the upper board 18 using asuitable adhesive 22. The stiffener board may or may not includeelectrical lines depending on the intended application. The flexibleoptical waveguide 12 is attached to the stiffener board 20 using asuitable adhesive 24 and is supported on a flexible substrate 26 thatpermits flexure of the flexible optical waveguide 12. A lower board 28is attached to the existing stack using a suitable adhesive. A connector30 is disposed at the board edge, which may be rigid or flexibledepending on the intended application.

FIG. 2 illustrates in greater detail a cross section of an exemplaryflexible optical waveguide 12, which is first fabricated as anindependent optical layer to enable the desired modularity. Thewaveguide includes a flexible substrate 40 using a laminated resincoated copper foil 42. The flexible substrate has the advantage in thatit allows waveguide layers to be laminated together using “roll to roll”manufacturing, in a process similar to newspaper printing, wherein oneor more flexible materials are applied from a roll. Exemplary flexiblematerials are polyimides, FR4, thin layer materials commonly used in PCBmanufacturing, e.g., rein coated copper, cooper, and the like.

Disposed thereon are a core layer 48 encased within cladding layers 44.A marker 46, e.g., a copper marker, is disposed in the under claddinglayer. The creation of markers, i.e., fiducials, serve as referencepoints and may be achieved using lithography techniques. The flexibleoptical waveguide is laminated to the PCB.

FIG. 3 illustrates an exemplary process for fabricating the flexibleoptical connector in the printed circuit board (PCB). The processincludes local lamination of the stiffener board 20 to the flexibleoptical waveguide 12. Local lamination may be provided by a structuredadhesive tape, bonding film, acrylate adhesive foil, patterned glue,patterned adhesion inhibitor layer, a sacrificial layer or anycombination thereof that permits local release as described. Thestiffener board 20 may or may not include electrical lines to partiallystiffen the flex, e.g., at termination. Optionally, a metallic stop maybe used to provide a stop layer on the stiffener board. The upperelectrical board 18 is then laminated to the existing stack containingthe flexible optical waveguide and stiffener board. The lower board 28is then laminated to the existing stack. FIGS. 4 and 5 illustrate theresulting structure after ia formation using laser drilling and platingprocesses along with trench milling, and laser cutting so as to provideoptical facet preparation. This is followed by flex contour millings andlaser cutting to permit flex release of the flexible optical waveguide.Connectors 30 can then be inserted using the passive alignment featuresin the flexible optical waveguide onto the ends of the flexible opticalwaveguide and then fixed. An opening is also milled on the top of theboard as shown more clearly in FIG. 5.

The termination can be part of a complex connector. The mechanicaladapter provides the alignment features. The mechanical transfer is astandardized optical connector that provides the alignment features. Inthe case of the mechanical transfer, there are two guiding pins (700micrometers and 4.6 millimeter center spacing) that are aligned with theoptical waveguide array in the same plane. At the end, the flexible;optical sheet is locally (close to the optical facet) released andequipped with a passively aligned connector. This results in amechanically decoupled opto-mechanical connector for the waveguides.

Any kind of optical element (active or passive) can be connected to theoptic-mechanical connector. By way of example, opto-electronic modulesmay contain optical transceivers, for example, vertical cavity surfaceemitting lasers (VCSELs) and photodiodes (PDs), which serve to transmitand receive optical signals, respectively. These modules can resideon/in the PCB, adjacent to/integrated with processors, applicationspecific integrated circuits (ASICs) and memory controllers, wheneverdense, high speed optical interconnects are required.

The modularity of the above noted systems given by functional separationof the interfaces provides benefits for designing and the reliability ofoptical interconnects. Modularity also enables individual improvement ofeach element, e.g., PCB, OE-subassembly, optics, thermal interfaces, andthe like. This approach not only simplifies the assembly process of theOE subassemblies to the PCB but it also advantageously providesincreased reliability due to reduced mechanical stress in the interface.Board openings as are typically required for opto-electronic subassemblycan be optimized to a smaller footprint and increased design freedom.This is especially advantageous for an application where theopto-electronic subassembly is attached below a carrier and thereforeprotrudes through the board. Accordingly, it is desired to have arelatively high I/O density located close to the opto-electronicssubassembly. This will serve to maximize the board are available forelectrical connections and enable a high degree of freedom for theelectrical design.

An exemplary use of the proposed mechanically decoupled connector isshown in FIG. 6. FIG. 6 illustrates a printed circuit board 50, thebackplane, whereby the released, flexible optical waveguide 12 is usedto provide a bend in the light path 54, e.g. a 90° bend with respect toteh waveguides in the backplane. Therefore, an optical connection to thewaveguides on the second printed circuit board 52, i.e. daughter card,is enabled. This bending enables an optical coupling to the waveguideson an additional printed circuit board, i.e., the daughter card.

The flow diagrams depicted herein are just examples. There may be manyvariations to these diagrams or the steps (or operations) describedtherein without departing from the spirit of the invention. Forinstance, the steps may be performed in a differing order, or steps maybe added, deleted or modified. All of these variations are considered apart of the claimed invention.

While the preferred embodiment to the invention has been described, itwill be understood that those skilled in the art, both now and in thefuture, may make various improvements and enhancements which fall withinthe scope of the claims which follow. These claims should be construedto maintain the proper protection for the invention first described.

1. A process for forming a printed circuit board including flexibleoptical interconnects, the process comprising: laminating a flexibleoptical waveguide to a stiffener board, wherein the flexible opticalwaveguide comprises passive alignment features, and wherein laminatingthe flexible optical waveguide provides portions of the flexible opticalwaveguide that are releasable from the stiffener board; laminating arigid circuit board to the stiffener board; laminating a lower board tothe flexible optical waveguide to form a laminated stack; patterning thelaminated stack to expose the portions of the flexible optical waveguidethat are releasable; and passively aligning a mechanical connector withthe passive alignment features and inserting the mechanical adapter ontothe portions of the flexible optical waveguide that are releasable.